LoadBalancerAutoConfiguration负载均衡自动配置里面用于传入负载均衡拦截器LoadBalancerInterceptor

spring 接口
package org.springframework.cloud.client.loadbalancer;

/**
 * Represents a client side load balancer
 * @author Spencer Gibb
 */
public interface LoadBalancerClient {

    ServiceInstance choose(String serviceId);

    <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException;

    URI reconstructURI(ServiceInstance instance, URI original);

}

一共就3个方法,

• 选择 选择具体一个服务实例
• 执行 执行请求
• 重新构造url 把url中原来写的服务名 换掉 换成实际的

只有 一实现类RibbonLoadBalancerClient 看里面的reconstructURI方法

/
 * @param instance 被选中的服务实例
     * @param original 原来写的url 里面直接写的服务名
     * @return 服务名替换掉的 实际url
     */
@Override
    public URI reconstructURI(ServiceInstance instance, URI original) {
        Assert.notNull(instance, "instance can not be null");
        
        String serviceId = instance.getServiceId();
        
        //clientFactory是 创造 负载均衡器 的工厂
        RibbonLoadBalancerContext context = this.clientFactory
        //每个ribbon客户端 都会给生成一个 spring上下文
                .getLoadBalancerContext(serviceId);
        
        //被选中的传入的服务实例 转成RibbonLoadBalancerContext需要的Server
        Server server = new Server(instance.getHost(), instance.getPort());
        boolean secure = isSecure(server, serviceId);
        URI uri = original;
        if (secure) {
            uri = UriComponentsBuilder.fromUri(uri).scheme("https").build().toUri();
        }
        //最终让全部交给RibbonLoadBalancerContext来干
        return context.reconstructURIWithServer(server, uri);
    }

RibbonLoadBalancerClient的execute 方法

    @Override
    public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {
        ILoadBalancer loadBalancer = getLoadBalancer(serviceId);
        //按一定策略选中具体的服务实例
        Server server = getServer(loadBalancer);
        if (server == null) {
            throw new IllegalStateException("No instances available for " + serviceId);
        }
        //服务实例包装一下 加入其他信息
        RibbonServer ribbonServer = new RibbonServer(serviceId, server, isSecure(server,
                serviceId), serverIntrospector(serviceId).getMetadata(server));

        RibbonLoadBalancerContext context = this.clientFactory
                .getLoadBalancerContext(serviceId);

//用来对服务的请求做跟踪记录的
        RibbonStatsRecorder statsRecorder = new RibbonStatsRecorder(context, server);

        try {
            //向这个具体的服务实例发起请求
            //apply方法接受一个ServiceInstance 是对一个服务实例的抽象定义,
//RibbonServer是对他的一个实现
            T returnVal = request.apply(ribbonServer);
            statsRecorder.recordStats(returnVal);
            return returnVal;
        }
        // catch IOException and rethrow so RestTemplate behaves correctly
        catch (IOException ex) {
            statsRecorder.recordStats(ex);
            throw ex;
        }
        catch (Exception ex) {
            statsRecorder.recordStats(ex);
            ReflectionUtils.rethrowRuntimeException(ex);
        }
        return null;
    }

首先就是 用serviceId得具体服务实例,怎么选择的由ILoadBalancer决定,这是Betflix Ribbon的接口 里面有个chooseServer方法

任何包装下Server 变为 RibbonServer 传入LoadBalancerRequest.apply(ServiceInstance instance) 发送请求

apply是定义在LoadBalancerInterceptor里面的匿名类的

@Override
public ClientHttpResponse apply(final ServiceInstance instance)throws Exception {

//ServiceRequestWrapper 是内部类  见下面    getURI()方法被重写了 
    HttpRequest serviceRequest = new ServiceRequestWrapper(request,instance);
    
    //里面会调用这里被重写的getURI()
    return execution.execute(serviceRequest, body);
}

private class ServiceRequestWrapper extends HttpRequestWrapper {

        private final ServiceInstance instance;

        public ServiceRequestWrapper(HttpRequest request, ServiceInstance instance) {
            super(request);
            this.instance = instance;
        }

        //这个方法被重写   用的是loadBalancer 也就是RibbonLoadBalancerClient 里面根据传入的服务实例改写原来的url
        @Override
        public URI getURI() {
            URI uri = LoadBalancerInterceptor.this.
            //之前说过的 reconstructURI 组织具体请求的服务器实例地址
            loadBalancer.reconstructURI(this.instance, getRequest().getURI());
            return uri;
        }

    }